The present invention relates to a brake. The present invention further relates to a hoist having incorporated such a brake.
It is generally known, especially in typical electricity meters, to generate for speed-dependent braking action a corresponding countertorque by means of an electrically conductive flat brake disk and a magnetic field which is produced by a horseshoe-shaped permanent magnet and oriented transversely to the brake disk. The magnetic field acts on a portion of the brake disk, whereby the brake disk moves through an air gap of the U-shaped permanent magnet. This leads in the brake disk to an inducement of a current which is approximately proportional to the rotational speed whereby the current circuit is closed across the portion of the brake disk that is not located in the magnetic field. The current flow generates an opposing magnetic field and, as a consequence of the resultant magnetic forces, the countertorque, to thereby effect a deceleration of the brake disk. The countertorque is dependent on the mean radial distance of the magnetic field from the rotation axis of the brake disk since the portion of the brake disk, penetrated by the magnetic field, moves, at constant rotational speed of the brake disk, through the magnetic field at growing radius and increasing velocity.
It is further known to provide drives for hoists for lifting and lowering loads with a squirrel-cage type induction motor as power driven and generator torque producing electric motor (so-called four-quadrant motor), and in addition with a brake, in particular to effect a delay and to hold a load. The rotational speed of squirrel-cage type induction motors is independent from the rotation direction of the brake disk and from the load, and generates by itself a desired braking torque when the load is lowered, i.e. when the load drives the motor. Typically, electromagnetic brakes or friction disk brakes are utilized as mechanical brakes.
It is an object of the present invention to provide an improved rotation-direction dependent brake which is cost-efficient, compact and wear-resistant.
It is another object of the present invention to provide an improved brake, which is applicable for a drive of a hoist, in particular for a drive having a motor, e.g. a cost-efficient universal motor, which does not produce a generator torque and/or runs at a highly load-dependent rotational speed as well as includes a self-locking gear mechanism.
These objects, and others which will become apparent hereinafter, are attained in accordance with the present invention by providing an electrically conducting brake disk rotating about a rotation axis; two magnetic poles positioned on both sides of the brake disk and defining a magnetic north and a magnetic south, for establishing a magnetic field directed transversely to the brake disk and acting, at least temporarily, on a portion of the brake disk, wherein the magnetic field has an orientation which depends on a rotation direction of the brake disk and generates a reactive force which retards the movement of the brake disk and is dependent from the rotation speed of the brake disk as well as from a radial distance from the rotation axis, wherein the magnetic poles are movable by the reactive force, in opposition to a return force applied by an elastic member, from a rest position along a predetermined path in rotation direction of the brake disk, wherein the radial distance is preset during movement along the path by a desired course of the rotation direction dependent and rotational speed dependent braking force and adjustable by the return force of the elastic member.
A brake according to the present invention is configured as eddy-current brake which is especially suitable for a rotary drive in the form of a dc motor or ac motor, without producing a generator torque and with highly load-dependent rotational speed, i.e. universal motors. In conjunction in particular with hoists, the use of such inexpensive universal motors is problematic because as a consequence of the lacking generator torque, universal motors cannot be decelerated in the normally simple way. Moreover, operation of universal motors is greatly dependent on the load and the rotational speed. When not subject to a load, universal motors exhibit a tendency to easily xe2x80x9coverspinxe2x80x9d, i.e. the rotational speed increases significantly. For that reason, universal motors should always run with a load. Moreover, the braking action should be dependent on the rotational speed, when used in hoists because a braking moment is only required during lowering of a load. Therefore, in accordance with the present invention, the magnetic poles are initially in a rest position. Only when the brake disk revolves will the magnetic poles move along the predetermined path as defined by a constraining guidance. The movement along this path is carried out in opposition to the return force applied by the elastic member, whereby the radial distance of the magnetic poles from the rotation axis of the brake disk varies during movement of the magnetic poles along the path. For example, the radial distance may steadily increase in proportional relation to the rotational speed of the brake disk, with the extent of the increase in both rotation directions being of different magnitude. During lifting, the braking moment of the brake disk can thus be set much smaller.
Suitably, the elastic member may be configured as a helical spring which automatically adjusts the predetermined radial distance with its spring tension according to the spring characteristic. This represents a simple solution and requires merely a selection of a helical spring of desired spring characteristic.
According to one aspect of the present invention, both magnetic poles may be located on a U-shaped permanent magnet.
According to another aspect of the present invention, the constraining guidance for conducting the magnetic poles along the predetermined path can be implemented by locating both magnetic poles on a free end of a swivel member which embraces the brake disk in a U-shaped manner and swings about a pivot axis extending in parallel relation to the rotation axis. Suitably, the swivel member swings at a pivot angle which is limited by a stop member. Thus, during swinging from a predetermined rotation speed, the greatest possible braking moment may be effective, with the magnetic poles positioned at a location of relatively great radial distance from the rotation axis.
The brake characteristic of the eddy-current brake can be configured for directional dependency by providing a second magnetic field which is aligned in a direction transversely to the brake disk and formed by further two magnetic poles. The provision of this second magnetic field is effective only in one rotation direction, i.e. in the event of a hoist only in the rotation direction that implements a lowering of a load.
According to another feature of the present invention, the swivel member includes two arms, with the magnetic poles of the two magnetic fields being located, respectively, on the free ends of both arms of the swivel member. A simplification can be realized when the second magnetic field acts only in one rotation direction of the brake disk and is located outside the brake disk when the brake disk rotates in the other rotation direction.
It is still another object of the present invention to provide an improved hoist which includes a simple brake.
This object is attained in accordance with the present invention by providing a lifting mechanism, which includes an electric motor configured as a dc motor or ac motor, which does not produce a generator torque and/or has a highly load-dependent rotation speed, and with a brake mechanism which is comprised of a self-locking gear mechanism, e.g. a worm gear, and an eddy-current brake. The eddy-current brake supports the gear mechanism which does not apply a braking action in the upper rotational speed range. At standstill, the statically self-locking gear mechanism solely provides the braking and holding functions.